I don't own a 1d mark III and I've never seen this in my 1Ds markII. A friend of mine is having the problem and I'm clueless as to what is causing it unless its a CF card issue. I believe it's happened on multiple CF cards, but they be identical cards.

Every once and a while, only the top left part of the image is recorded, probably about 50% of the total image ,the rest of the image area is just white. This is happening when he is working pretty slowly (fall leaves on tripod), not an issue of filling up a buffer, or card writing too slow.

Any ideas? I'll try to get a sample image to show what I'm talking about, but thought I'd ask in case someone recognizes the problem...

I had a shutter problem some time back with my 1DsII, where the top of the frame wasn't recording at the higher shutter speeds. Canon replaced the shutter. This almost sounds like the opposite, but maybe it's some shutter synchronization problem?